This invention relates generally to anesthesia machines and more particularly to safety devices therefore, and is a Continuation-In-Part of my co-pending United States Application Ser. No. 086,196 filed on Oct. 18, 1979 now abandoned and whose disclosure is incorporated by reference herein.
As is known, commercially available rebreathing type anesthesia machines basically comprise means for supplying fresh gas, i.e., oxygen and an anesthesia gas, through conduits for introduction into the patient's lungs and rebreathing means in the form of an inflatable component to serve as a counterlung. Typical of such counterlungs are inflatable breathing bags and ventilator bellows. The rebreathing means is coupled through a CO.sub.2 absorber or cannister to the fresh gas supply. An inspiratory valve is coupled to the gas supply means and to the rebreathing means to enable gas from the fresh gas supply and the counterlung to flow into the patient's lungs during the inspiratory phase of a breathing cycle. An expiratory valve enables the gas expelled from the patient's lungs to flow to the counterlung. A valve, commonly called an adjustable pressure limiting (APL) valve is coupled to the rebreathing means and arranged to vent expired gas out of the system when the gas pressure slightly exceeds the inflating pressure of the breathing bag.
Rebreathing type anesthesia apparatus may be operated in one of three different modes of operation, namely, spontaneously breathing, artificial ventilation by means of manual bag squeezing and artificial ventilation by the use of a powered ventilator.
During spontaneous breathing, the lungs of the patient form the active member for moving the gas within the system while the breathing bag acts as a passive flexible member. A selector valve is used to connect the patient's breathing system to the rebreathing bag. During the inspiratory phase of spontaneous breathing, the patient expands his thorax, thereby generating a sub-atmospheric pressure of approximately 1 cm of water, which draws gas from the partially or completely filled rebreathing bag, via the selector valve, the carbon dioxide absorber and the inspiratory valve, into the lungs. During the same inspiratory period, the fresh gas supply means feeds a continuing gas flow into the system. This continuing flow joins the gas from the rebreathing bag on its way to the patient's lungs. The amount of fresh gas provided into the system is determined by the anesthesiologist, but must at least contain the metabolic oxygen requirements of the patient and the volume of anesthesic gases or vapors which is required to maintain anesthesia. Based on safety considerations, the amount of gas delivered into the system is usually many times the amount of volume per minute actually required by the oxygen consumption and anesthesia needs of the patient. If the breathing circuit includes various accessories, such as, humidifiers, bacteria, filters, etc., the resistance of the breathing circuit is increased and a higher sub-atmospheric pressure may be produced by the patient during the inspiratory phase, with the maximum acceptable sub-atmospheric pressure being approximately 2 cm of water.
At the end of the inspiratory phase, there is a short pause during which the breathing circuit pressure is atmospheric. As the expiratory phase commences, the pressure begins to increase above atmospheric pressure. During the expiratory phase, the patient exhales from his lungs, via the expiratory valve and the selector valve, into the rebreathing bag.
Inasmuch as fresh gas is delivered into the system by the fresh gas supply, during both the inspiratory and expiratory phases, the total amount of gas in the system is greater toward the end of the expiratory cycle. Accordingly, at a certain point in the expiratory phase, the rebreathing bag will be filled while the pressure in the breathing system continues to increase until the APL valve opens. The APL valve, when open, serves a bleed off gas into a gas scavaging system to prevent the build up of excess pressure in the breathing system.
APL valves vary in their design and specific performance. Some APL valves are spring loaded devices which remain closed until subjected to a predetermined threshold pressure at which time they open fully. Other APL valves are of the resistance type wherein the flow resistance of the valve is established by the adjustment of a variable orifice. Such valves constantly bleed off gas from the breathing system. Irrespective of the type of APL valve used, the valve is adjusted so that it operates to bleed off gas from the breathing system cycle, each cycle to prevent the pressure therein from exceeding slightly more than the inflating pressure of the counterlung, e.g., bag. To that end, the APL valve may be set to prevent the pressure from exceeding 1-3 cm of water, depending upon conditions. Thus, the amount of gas expelled through the APL valve depends upon and is approximately equal to the amount of fresh gas delivered from the anesthesia machine during the same period of time. While anesthesia apparatus include pressure gauges for displaying system pressure, due to the extremely low pressure setting of the APL valve, the very slight pressure fluctuations during spontaneous breathing are normally not observable at the pressure gauge.
During artificial ventilation by means of bag squeezing, the selector valve of the apparatus is set in the same position as during spontaneous breathing, namely, to connect the rebreathing bag to the breathing system.
In the bag squeezing operation, in order to perform the ventilation, the pressure setting of the APL valve is increased substantially to a level equal to the maximum of inspiratory pressure required to ventilate the patient adequately. Such a pressure setting may depend on the conditions of airway resistance and lung compliance.
Operation of the breathing system during bag squeezing is as follows: the partial or completely filled rebreathing bag is squeezed manually, thus increasing the pressure within the breathing system. The increased pressure in the system creates a flow of gas from the system through the airway to the lungs through the same path as occurs during the inspiratory phase of spontaneous breathing, with the pressure in the system being always higher than the pressure in the lungs during the inspiratory phase (depending on the airway resistance).
In anesthesia machines using spring loaded APL valves, the pressure in the system rises until the opening pressure of the APL valve is reached and thereafter the gas is expelled or bled through the valve into a scavenging system. This precludes further increase in breathing system pressure.
During either the spontaneous breathing or artificial ventilation by bag squeezing modes of operation, the expiratory phase results from the simple release of pressure on the rebreathing bag. This permits the patient to spontaneously exhale from his lungs, via the expiratory valve and the selector valve into the rebreathing bag.
During artificial ventilation employing a ventilator, e.g., bellows, operation of the system is as follows: the selector valve is set so that the bellows of the ventilator is connected to the breathing system and the rebreathing bag disconnected from the breathing system. In this mode of operation, the bellows is contracted, thus delivering gas via the selector valve, the CO.sub.2 absorber and the inspiratory valve to the patient's lungs. In order to bleed excess gas from the ventilator, the ventilator includes a relief valve which is closed by means of pressure in a pilot line leading from the ventilator.
As in other modes of operation, the flow of fresh gas is delivered continuously into the patient breathing system. The continuing fresh gas flow joins the inspiratory gas delivered from the bellows on its way to the lungs of the patient. With the beginning of the expiratory phase, the pressure on the bellows is released, thus permitting the bellows to expand and the patient to exhale from his lungs, via the expiratory valve and the selector valve, into the ventilator bellows. At the same time, the pilot pressure which closes the ventilator relief valve during the inspiratory cycle decreases to zero and permits the ventilator relief valve to be opened by the higher pressure within the breathing system. Thus, an amount of fresh gas equal to the amount delivered by the anesthesia machine during the inspiratory-expiratory cycle is expelled into the scavenger system of the anesthesia machine.
In both types of artificial ventilation, i.e., manual bag squeezing or automatic ventilation by a bellows, it is absolutely necessary for the operator to insure that there is produced a positive pressure in the breathing system in order to ventilate the patient's paralyzed lungs. Positive pressure in the lungs, however, retards the return of venous blood flow, which may leave vital organs, like the brain, without necessary oxygen supply, thereby exposing the patient to severe injury. Thus, during operation of an anesthesia machine in either the spontaneous breathing mode of operation or in either of the two artificial ventilation modes of operation, it is of considerable importance that an anesthesia machine be operated so that it does not maintain positive pressure for an abnormally long period of time. In normal operation, the alternating pressure changes during the inspiratory and expiratory phases permit the venous return of blood during periods of low pressure.
Various accidents have been reported using ventilating apparatus due to operator error resulting in the production of continuing positive pressure in the system. For example, one typical operator-caused type of accident occurs as follows: During surgery while a patient's lungs are disabled and the machine is operated in the bellows ventilating mode, the operator closes the APL valve. At the end of the surgical procedure when the patient regains his capability to spontaneous breathing, the operator switches the selector valve back to the setting for connecting the rebreathing bag, but due to oversight or misjudgment, the anesthesiologist or operator neglects to open the APL valve. This results in excessive pressure in the system (since fresh gas is being continuously delivered into the system and cannot escape through the closed valve) and patient injury may result.
Penlon Ltd. of Radley Road, Abington, Oxon OX143PH England manufactures an IDP Pressure Failure Alarm which is a self-contained battery powered alarm designed for patient protection against ventilator failure or circuit disconnection during Intermittent Positive Pressure Ventilation (IPPU). When the alarm is connected to the breathing circuit of an anesthesia apparatus, it produces an audible and visual warning if the pressure in the breathing circuit fails to increase through a set level, e.g., 7.5-12.5 cm of water, and then decreases through the set level within a predetermined period of time.
While the Penlon device appears suitable for its intended purposes during either artificial mode of ventilation, it is incapable of use during the spontaneous mode of operation. This drawback is due to the fact that the Penlon device will produce an alarm at all times during spontaneous breathing since the pressure fluctuations during spontaneous breathing are substantially below the threshold of the Penlon device.
In U.S. Pat. No. 3,333,584, there is disclosed a pressure breathing monitor which like the aforementioned Penlon device, appears incapable of use in spontaneous breathing conditions.
In our aforementioned parent patent application, there is disclosed and claimed a safety system for anesthesia apparatus, which produces an express warning to the operator of the existence of continuous pressure within the patient breathing system. To accomplish that end, the system includes means for monitoring the pressure within the patient breathing circuit and for providing a warning signal to the operating personnel in the event that the system pressure is sustained for a predetermined period of time and irrespective of the mode of operation of the anesthesia apparatus.
Those and other objects of the invention disclosed and claimed in that application are achieved by providing safety means for use in an anesthesia apparatus. The anesthesia apparatus includes a breathing circuit comprising gas supply means and rebreathing means comprising alternately selectable powered ventilator means and rebreathing bag means and means for selecting the rebreathing means. The apparatus is arranged for supplying gas to a spontaneously breathing patient through the breathing bag means during a first mode of operation or to artificially ventilate the patient by manually squeezing said bag means during a second mode of operation or to artificially ventilate the patient automatically by powered ventilator means during a third mode of operation. The gas supply means and the selected rebreathing means are connected together by the selecting means for enabling gas from the gas supply means and gas from the selected rebreathing means to flow together at a first point. Conduit means are coupled to the lungs of the patient. Inspiratory valve means are coupled to the gas supply means and the rebreathing means for enabling gas from the first point to flow downstream through the conduit means to the patient's lungs. Expiratory valve means are coupled between the conduit means and the rebreathing means for enabling gas from the first point to flow downstream through the conduit means to the patient's lungs. The expiratory valve means are coupled between the conduit means and the rebreathing means for enabling gas to flow downstream from the patient's lungs through the conduit means to the selected rebreathing means. Pressure release means are provided to venting gas out of the breathing circuit. The safety means comprises pressure sensing means for monitoring the gas pressure downstream of the first point and means coupled to the pressure sensing means and responsive thereto for providing an alarm signal only in the event that the gas pressure monitored exceeds a predetermined value for a predetermined period of time and irrespective of the mode of operation of said apparatus.
The instant invention comprises an improvement over the invention disclosed and claimed in our aforementioned parent patent application by providing a safety system which exhibits all of the features and advantages of that invention and which also ensures that there is no excessive power drain on the electrical power source for the safety system.